The present invention relates to an image forming apparatus such as an electrophotographic copying apparatus and a printing apparatus for forming an image by successively forming a plurality of toner images different in color on a photosensitive drum and transferring the plurality of toner images formed on the photosensitive drum in such a manner as to superpose the toner images successively on a transfer material held on a transfer drum, and more particularly to the art of preventing nonconformity detecting means for detecting the transfer material conveyed to the transfer drum in nonconforming condition from making any detection error.
A typical image forming apparatus of the sort which employs a transfer drum is used for forming an image by successively forming a plurality of toner images different in color, for example, images of cyan, magenta, yellow, black and the like, on a photosensitive drum and transferring the plurality of toner images formed on the photosensitive drum so as to superpose the toner images successively on a transfer material such as transfer paper held on a transfer drum. In the image forming apparatus, a predetermined number of toner images are transferred in a multiplex mode onto the transfer material absorbed onto the transfer drum and then the transfer material is peeled off the surface of the transfer drum, whereby the toner image is fixed on the transfer material by means of a fixing unit. In such an image forming apparatus, further, a copy can be made on both sides of the transfer material by turning the transfer material with the toner image formed on one side upside down and feeding it again.
In the image forming apparatus, a nonconformity detecting means is also used for detecting whether the transfer material is properly held on the surface of the transfer drum or whether the transfer material has been peeled off the surface of the transfer drum for certain. In the image forming apparatus, further, a cleaning unit is used for cleaning the surface of the transfer drum because floating toner, the extraneous additive of the toner or paper powder originating from transfer paper as the transfer material tend to stick to the surface of the transfer drum.
A transfer material 101 supplied from a paper feed cassette (not shown) is pressed by an absorption roll 102 against the surface of the transfer drum 100 as shown in FIG. 14. The transfer material 101 is charged by an absorption corotron 103 from the back side of the transfer drum 100 and electrostatically absorbed. Further, toner images formed on a photosensitive drum 104 are successively transferred onto the transfer material 101 absorbed onto the surface of the transfer drum 100, and the transfer material 101 is peeled by a peeling unit 105 from the surface of the transfer drum 100. The toner image is then fixed on the transfer material 101 by a fixing unit 106. Thus the image-forming process is terminated.
In that case, jam sensors 107, 108, 109 are arranged on the outer periphery of the transfer drum 100: the jam sensor 107 installed on the downstream side of the absorption roll 102 is used for detecting whether the transfer material 101 is absorbed onto the surface of the transfer drum 100; the jam sensor 108 installed on the downstream side of the photosensitive drum 104 and its nip portion is used for detecting whether the transfer material 101 is held on the surface of the transfer drum 100; and the jam sensor 109 installed on the downstream side of the peeling unit 105 is used for detecting whether the transfer material 101 has been peeled off the surface of the transfer drum 100. As disclosed in Unexamined Japanese Patent Publication 62-280883/(1987), for example, a cleaning brush 110 for cleaning the surface of the transfer drum 100 is arranged on the outer periphery of the transfer drum 100.
However, there is a problem arising from the prior art as follows: When the transfer material 101 with the plurality of toner images different in color, that is, toner images of cyan, magenta, yellow, black and the like transferred thereto is fixed by the fixing unit 106 in the case of the conventional image forming apparatus, a releasing agent such as silicone oil greater in quantity than what is used for a normal black and white copy is used to restrain the toner from being offset on the transfer material 101, and the releasing agent also sticks onto the transfer material 101. For this reason, the surface of the transfer material 101 on which the toner image has been formed may touch that of the transfer drum 100 particularly when both-side printing is made in the image forming apparatus of the sort mentioned above and this may cause the releasing agent sticking onto the transfer material 101 to stick to the surface of the transfer drum 100 likewise. Then the releasing agent sticking to the surface of the transfer drum 100 acts as an adhesive and this allows the floating toner and the extraneous additive of the toner or otherwise paper powder originating from transfer paper as the transfer material 101, for example, to stick onto the surface of the transfer drum 100.
Like this, the floating toner and the extraneous additive of the toner or otherwise the transfer paper powder produced from the transfer material 101, particularly the extraneous additive of the toner and the paper powder having a small particle size are impossible to remove completely by the cleaning brush 110 and as they gradually accumulate on the surface of the transfer drum 100, thus making the surface of the transfer drum 100 become opaque. When the surface of the transfer drum 100 becomes opaque, the detection level of jam sensors 107, 108, 109 disposed on the outer periphery of the transfer drum 100 lowers, thus causing the jam sensors to make a detection error. Consequently, the problem is that a replacement interval of a transfer film 100a is shortened because the equipment may stop operating properly unless the transfer film 100a on the surface of the transfer drum 100 is replaced. This problem is found noticeable particularly when a copy is made on both sides of the transfer material in the image forming apparatus above. However, the extraneous additive of the tone and the paper powder gradually accumulate on the surface of the transfer drum even when both-side printing is not made and the surface of the transfer drum gradually becomes opaque likewise, which also poses a similar problem.
In the image forming apparatus using such a transfer drum, moreover, use may also be made of a concentration detecting means for detecting the toner concentration of a test patch on the transfer drum 100 by transferring the test patch onto the transfer drum 100 as occasion demands in addition to the transfer material detecting means such as the jam sensor 107 and so on for detecting the conditions in which the transfer material 101 is conveyed on the transfer drum 100. The opaque condition of the surface of the transfer drum 100 may become the cause of a detection error at the time the toner concentration of the test patch is detected.